This movie was not funny because of its poor treatment of the subject matter.
It would be difficult to make a funny movie about the KKK and, without trivializing the evil of the KKK, romantic obsessions have killed a lot more people. Better movies like Strangers on a Train succeed at depicting killing your spouse because comic moments are balanced with an authentic appreciation of the horror.
Also horrifying, is the number of people who think this movie had a positive message and think the real life couple enjoyed a happy ending. Trying to kill someone is not a sign of love but of selfish attachment. As Gloria Steinem said: love is wanting the best for another person, romance is just wanting the other person.